A mechanical model of the four-point end notched flexure (4ENF) test based on an elastic-brittle interface

The paper introduces a mechanical model of the four-point end notched flexure (4ENF) test used to assess the mode II interlaminar fracture toughness in laminated specimens under stable crack-growth conditions. The model considers the specimen as an assemblage of two sublaminates, partly bonded together by a deformable interface. Each sublaminate is modelled as an elastic orthotropic beam, while the interface consists of a continuous distribution of normal and tangential linearly elastic-brittle springs. The mechanical behaviour of the system is described by a set of twenty-four differential equations, endowed with suitable boundary conditions. The original problem is split into two sub-problems, considering separately the symmetric and antisymmetric loads. The explicit solution to the problem is deduced for the internal forces and interlaminar stresses. Moreover, the energy release rate and compliance are determined. The predictions of the model are compared to theoretical and experimental results available in the literature.